The interleukin-2 receptor and related cytokine/cytokine receptor systems are being studied to understand critical components of the T cell immune response in normal and neoplastic cells. After T-cell activation, IL-2 and IL-2 receptors are induced; the magnitude and duration of the T-cell immune response is controlled by the amount of IL-2 produced, the levels of receptors expressed, and the time course of these events. Expression of IL-2Ra is interestingly high in cells infected with HTLV-I, the cause of adult T cell leukemia (ATL) and tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). Three chains of the IL-2 receptor exist, IL-2Ra, IL-2Rb, and gc, with IL-2Ra and IL-2Rb being regulated at the level of transcription. gc is a shared chain also used by the receptors for IL-4, IL-7, IL-9, IL-15, and IL-21, and is the protein that is mutated in XSCID. We have focused primarily on the types of signals induced by these cytokines, particularly the activation of STAT proteins (signal transducers and activators of transcription), and the mechanism by which they regulate target genes. Given our prior data that STAT5A or STAT5B transgenic mice develop tumors, consistent with STAT5 being implicated in malignant transformation and elevated in a range of human tumors, this is an important area for both normal and pathological states. Moreover, humans and mice with defective STAT protein expression have a range of immunological defects. T helper cell differentiation is critical for normal immune responses, with Th1 differentiation being important for host defense to viruses and other intracelllular pathogens, Th2 differentiation being vital in allergic disorders and related to helminths, and Th17 differentiation being vital in a range of inflammatory disorders, including psoriasis and inflammatory bowel disease. We previously showed that IL-2 is important for Th2 differentiation and that IL-2 induces IL-4 receptor expression in a STAT5-dependent manner and critically controls priming of cells for Th2 differentiation. Moreover, using genome-wide chromatin immunoprecipitation coupled to DNA sequencing (ChIP-Seq) analysis, we previously found broad regulation of Th2 differentiation via STAT5A and STAT5B. We previously extended these findings by showing that IL-2 via STAT5 induces expression of IL-12Rb2, which is critical for Th1 differentiation. Additionally, we showed that IL-2 via STAT5 also regulates the T box protein, T-bet. Interestingly, IL-2 also inhibits expression of IL-6Ra and gp130, helping to explain the inhibition of Th17 differentiation. These results indicated a very broad effect of IL-2 via STAT5 on T helper cell differentation. In the current review year, we continued to study the role of IL-2 in Th differentiation and published a major paper reporting the critical role of IL-2 in Th9 differentiation. We demonstrated a direct effect of IL-2 on Th9 differentiation via its induction of STAT5 binding to the Il9 promoter. Moreover, we showed opposing actions of IL-2 and IL-21 in Th9 differentiation based on their differential regulation of BCL6, which is induced by IL-21 but repressed by IL-2. We also have continued our studies of the role of STAT5 tetramerization in vivo. We previously collaborated with Dr. K. Christopher Garcia at Stanford on a project in which the three dimensional structure of IL-2 complexed to its receptor was compared to that of IL-15 bound to its receptor. These studies had been published in Nature Immunology and provided key mechanistic and structural insights into the functional differences between IL-2 and IL-15. In the current year, we continued our collaboration with Dr. Garcia, studying the actions of wild type IL-2 versus novel IL-2 variants, a project with potential clinical ramifications. Some of these IL-2 variants are potent inhibitors of both IL-2 and IL-15, and some have partial agonistic activity. Much more work is required, but it is possible that these may prove to be valuable clinically, potentially in cancer and in immunological diseases. Moreover, the approach used to generate these novel IL-2 variants should be broadly applicable to other gc family cytokines and potentially other type 1 cytokines as well. Previously, we demonstrated that IL-21 regulated expression of the Prdm1 gene that encodes BLIMP1 via a response element that depends on STAT3 and IRF4 and subsequently discovered that in contrast to its known ability to cooperate with PU.1 in B cells to act via Ets-IRF composite elements (EICEs), IRF4 cooperates with BATF/JUN family proteins to act via AP1-IRF composite elements (AICEs) in T cells, as well as in B cells. We demonstrated critical cooperative regulation of important genes via these AICEs and demonstrated cooperative binding of IRF4, BATF, and JUN family proteins, with markedly diminished IRF4 binding in Batf-deficient cells and markedly diminished BATF binding in Irf4-deficient cells. We demonstrated critical regulation of key genes, including for example those encoding IL-10 and IL-17 via AICEs. We have continued our studies of AICEs and IRF4/BATF/JUN/STAT3 complexes. IL-21 has broad actions on T- and B-cells, but its actions in innate immunity are poorly understood. We previously reported that IL-21 induced apoptosis of conventional dendritic cells (cDCs) occurs via STAT3 and Bim, and this was inhibited by granulocyte-macrophage colony-stimulating factor (GM-CSF). ChIP-Seq analysis revealed genome-wide binding competition between GM-CSF-induced STAT5 and IL-21-induced STAT3. We have continued our studies of the actions of IL-21 on DCs and the importance in the immune response. Overall, the above findings enhance our understanding of mechanisms by which the gc family cytokines regulate gene expression and biologically important processes. In addition, these findings have implications related to the treatment of cancer, autoimmune, and other diseases.